This invention is related to a vehicle drive system and its controlling method.
In recent years, due to demand for low pollution, energy saving and better infrastructure, attempts have been made to commercialize hybrid vehicles wherein an engine is combined with a generator and a motor.
Hybrid vehicle systems may be broadly classified as series or parallel types, and in both cases, all or part of the kinetic energy of the engine is converted to electrical energy by the generator. A part of the power produced by the generator is directly supplied to the motor, and the excess power is stored by a battery. Power is supplied to the motor from both the generator and the battery so as to drive the vehicle. Losses are not small when the energy produced by the engine passes through the generator, battery and motor, so the parallel type of hybrid vehicle, which comprises a means for directly driving the vehicle by engine power and has high efficiency, is now the most popular.
However, even in the series type, losses when the battery is charged or discharged can be largely reduced and an increase in efficiency can be expected if the power consigned by the motor is supplied from the generator without excess or insufficiency.
In this regard, JP-A-H11-146503 and JP-A-2000-236602 published by the Japanese Patent Office in 1999 propose a means of increasing efficiency of a series type hybrid vehicle by generating the required power based on the vehicle running state. The drive power (power consumption) of the motor varies according to the change of running state, but if power can be supplied from the generator in real-time without excess or insufficiency according to this variation, the power losses in the battery can be kept to the minimum, and the output of the engine can be transmitted to the motor with high efficiency.
However, when the power consumption of the motor varies sharply due to a sudden change of running state, a delay occurs in power generation due to the response delay of the engine and motor even if the engine and motor are controlled to make the generated power follow the change, so the generated power is then insufficient or excessive relative to the power consumption of the motor.
Regarding this point, one way of suppressing power losses in the battery might be to estimate the actual power generated based on the running state of the engine and motor, limit the power of the motor based on the estimated value and suppress the power difference between the power consumption of the motor and the power generated by the generator even in a transient state.
However, even under this control, when the battery temperature is correct and the battery is sufficiently charged, it may be advantageous not to supply all the power required by the motor from the generator in real-time, but to supply power to the motor from the battery. In other words, if fuel cost-performance can be improved regardless of the charge/discharge power losses of the battery, it is advantageous to run the vehicle steadily at an operating point where the engine is more efficient or restrict engine power, and make up the power deficiency from the battery.
In this case, however, it may occur that the power generated by the generator is less than the power consumption of the motor. Hence, if the power of the generator is limited according to the actual generated power as described above, the power of the motor is limited to low power when the power generated by the generator is low even in the same running state, and the output response of the motor decreases.
It is therefore an object of this invention to make the output response of the motor the same regardless of the power generated by the generator and power supplied by the battery in the same running state.
In order to achieve above object, this invention provides a drive system for a vehicle, comprising an engine, a generator connected to the engine, which regenerates the output of the engine, a motor for driving the vehicle which operates by power supplied from the generator, a battery which supplies power to the motor when the power supply to the motor is insufficient, and stores excess power when the supplied power is excessive, a sensor which detects a vehicle running state, and a controller which functions to compute a virtual generated power, which is the power generated by the generator when the engine and generator are controlled so that the power generated by the generator coincides with the power consumption of the motor which is uniquely determined according to the vehicle rung state, compute an output limiting value of the motor based on the virtual generated power, and limit the power or torque of the motor by the output limiting value.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.